This invention relates to the field of cardiac prosthetic devices and, more particularly, to an electronic control system for an artificial heart and circulatory assist device, especially for use by and implantation in humans.
It has been estimated that between 16,000 and 50,000 patients annually are suitable candidates for implantation of a total cardiac prosthesis (TCP). Such candidates typically are disabled due to insufficient left and right ventricular function, but are otherwise in good health. Many thousands more annually, having inadequate left ventricular function and satisfactory right ventricular function, may be candidates for a permanently implanted left ventricular assist device (LVAD).
The ideal total cardiac prosthesis must provide complete rehabilitation for the patient. Such a TCP recipient must be able to engage in gainful employment and all normal activities, including moderate exercise. He should retain a substantially normal appearance and normal or near normal mobility with no significant limitations of any kind. Cardiac output effected by the TCP must be normal, adequat and sufficiently responsive to the patient's requirements to accommodate expected, sudden changes in physical activity or emotional stress level. The presence and operation of the TCP must be sufficiently unobtrusive so that the patient can largely forget that he is dependent on an artificial heart. All blood pumping functions of the TCP should be completely automatic, so that the patient performs no control or monitoring functions except for maintaining adequate power to the TCP and responding to warnings that indicate a lack of power or serious problems requiring immediate technical or medical attention.
The intrathoracic blood pumping components of the TCP must be similar in size and weight to the natural heart. TCP life must be sufficiently long and reliability sufficiently high that risk to the patient of sudden prosthesis failure and its attendant anxiety are minimized. The formation of pannus and adherent thrombus must be prevented to avoid a compromise of blood pump function. Thrombo-emboli and excessive blood damage also must be prevented. The TCP must not damage adjacent tissues or impair organ function by toxicity and adverse tissue reactions, by mechanical trauma or compression, or by excessive local temperatures. The system must avoid skin penetrations of any kind to prevent infections that can arise from percutaneous leads. This eliminates a major risk to the patient, reduces the need for clinical observation and treatment, and reduces the maintenance of the TCP required of the patient. This ideal system must be low in cost to purchase, implant and maintain. The frequency and extent of routine monitoring and maintenance, both medical and technical, must be low.
Serious research toward the realization of a total cardiac prosthesis has been under way since about 1957, sponsored largely by the U.S. National Institute of Health (NIH). Researchers have directed this research to six principal areas: (1) blood-compatible materials for the blood pumping device; (2) heart valves; (3) blood pumps; (4) blood pump actuating devices; (5) power supplies and their application to the internal blood pump actuating device; and (6) control mechanisms for the pumping function.
The prior related applications Ser. No. 265,100 and Ser. No. 265,199 discuss previous research and development in blood-compatible materials, heart valves, blood pumps and blood pump actuating devices. The discussion in these applications is hereby incorporated by reference in this application. In addition, these prior related applications are principally directed to an hydraulically actuated blood pump and blood pump actuating device. This hydraulically actuated system includes an actuation fluid reservoir, an actuation fluid pump in fluid communication with the reservoir for providing intermittent pulses of actuation fluid and an actuation chamber having an actuation fluid inlet path in fluid communication with the pump and a separate actuation fluid outlet path in fluid communication with the reservoir. The actuation chamber causes displacement of a flexible portion or bladder of a blood pumping chamber in response to changes in the volume of actuation fluid in the actuation chamber. The hydraulically actuated system also includes a valve associated with the actuation chamber to close or open the actuation fluid outlet ath primarily in response to forces which vary as a function of actuation fluid flow through the actuation fluid inlet path. The operation of the valve, the actuation fluid pump and the actuation chamber expels blood from the blood pumping chamber after the blood pumping chamber fills with blood. Thus, these prior related applications disclose a practical blood pump actuating mechanism which, for the sake of convenience, is incorporated in this application.
On the subject of power, up to this time, most TCPs implanted in calves have been powered pneumatically via transcutaneous tubing into the thoracic cavity. A large external console supplies the proper regimen of pressure variations in order to activate the internal blood pump. With such a system, calves have lived up to 221 days (Jarvik, "The Total Artificial Heart," Scientific American, Vol. 244, No. 1, pp. 74-80, January, 1981). On another tack, NIH has sponsored considerable effort on the development of internal nuclear power supplies and, to a lesser extent, of chemical fuel cells. None of this work, however, appears to be promising; in fact, the nuclear effort was terminated by the U.S. Energy Research and Development Administration. Additionally, various means of transmitting mechanical power transcutaneously have been attempted, but none appears to be promising. At present, transcutaneous transmission of electricity appears to be the preferred method for powering a TCP. A second, less preferable possibility is the supplying of electrical power through percutaneous wire penetrations, but these always pose a threat of infection and are psychologically annoying to the patient.
Several investigators have developed the technique of transcutaneous electrical transmission. Their approach is to implant a coil under the skin. This coil functions as a transformer secondary winding, receiving power from an inductively coupled, external mating coil juxtaposed therewith to serve as the transformer primary winding. At frequencies on the order of 17 kHz, up to 100 watts of power have been transmitted for many months across the skin of a dog. See J. C. Schuder et al, "Ultra High Power Electromagnetic Energy Transport Into the Body," Trans. ASAIO, 1971, incorporated herein by reference. Thus, the inductive delivery across the intact skin of approximately 30 watts of power to a TCP appears to be well within the state of the art.
On the subject of control of a TCP to make it sympathetic to the body, there have been many different approaches and much controversy. Some researchers have attempted to provide no active control, while others have required control in order to achieve regular beating. See, e.g., W. H. Burns et al, "The Total Mechanical Cardiac Substitute," Process in Cardiovascular Diseases, Vol. XII, No. 3, 1969, pp. 302-311. Some systems have attempted to control systole (i.e., the contraction phase of the cardiac cycle whose rate is one determinant of cardiac output) from the left ventricle of the TCP in order to control the systolic pressure in the aorta. Still other systems have attempted feedback control of stroke volume and beat rate.
The natural heart and at least some, if not all, TCPs are comprised of two pumps in series. The right pump or ventricle receives blood from the vena cava and ejects it into the pulmonary artery. The left pump or ventricle receives blood from the pulmonary veins and ejects blood into the systemic circulatory system via the aorta. Over a time period considerably longer than that of a few beats, the left pump will pump more blood than the right pump. To prevent an imbalance of blood volume, a deficiency or excess of blood pumped by the right and left ventricles must be avoided. Various investigators have included controls in their TCP systems in order to achieve the critical balance between the pumping rate of the right and left ventricles. The major intrinsic mechanism by which the natural heart controls cardiac output is described by Starling's law of the heart, which essentially states that the output during systole is proportional to the amount of blood which flows into the relaxed ventricle during diastole. The body controls peripheral vascular resistance and venous "tone" according to the needs of the body's organs so that blood flowing to, and venous return from, the body is increased when there is a demand for higher blood flow. This increased return is accompanied by increases in venous and atrial filling pressures normally ranging from 0 to 10 mm Hg, which drives blood from the vena cava through the tricuspid valve into the relaxed right ventricle during diastole.
Similarly, for the left ventricle, the pressure in the pulmonary veins and left strium normally varies from 5 to 15 mm Hg and is proportional to venous return from the pulmonary vascular network into the left ventricle. If the right ventricle should temporarily pump slightly more than the left ventricle, the pressure rises in the pulmonary artery and, as a consequence, in the pulmonary veins, causing more blood to flow into the left ventricle and thereby matching the pumping output of the left ventricle to that of the right ventricle. Thus, the natural heart achieves the necessary balance between the two pumps in series via simple and direct fluid dynamic means. In a real sense, the heart is the servant, not the master of the circulatory system. It is basically just a pump which must respond to the requirements of the body by pumping precisely that which returns to it. The above-described intrinsic control can maintain body function even in the absence of extrinsic humoral or neural control.
The body also neurally controls the rate at which the natural heart beats. Cardiac output is a function of the amount of blood ejected during systole, and the rate at which the heart beats. For all but the most strenuous activity, the systolic stroke volume per beat remains substantially constant. Thus, cardiac output is primarily a function substantially constant. Thus, cardiac output is primarily a function of heart rate (i.e., the number of beats per minute). Heart rates can vary from a low of about 40 to as high as 220 beats per minute in a young person, and ordinarily from about 60 to 150 bpm in an adult. Cardiac output of the natural heart can vary from about 4 to as high as 24 liters per minute, the latter being the case of a trained athlete. Experience with transplanted natural hearts shows that direct neural control is unnecessary for a satisfactory life. However, transplanted hearts do respond indirectly to neural commands since the peripheral vascular system and venous return to the heart are regulated by the nervous system.
The natural control system also regulates arterial blood pressure in order to maintain adequate circulation to the vital organs. The mean arterial pressure is established by cardiac output and the peripheral resistance of the vascular systems. In some of the TCPs which previously have been developed, a control means has been provided to maintain pressure in the aorta and atrium within a reasonable range. Pierce et al, "Automatic Control of the Artificial Heart," Trans. Amer. Soc. Artif. Int. Organs, Vol. II, pp. 347-356 (1976); Kirby W. Hiller et al, "An Electronic-Mechanical Control for an Intrathoracic Artificial Heart," American Journal of Medical Electronics, July-September of 1963, pp. 212-221; Hiller et al, "Mechanism to Drive An Artificial Heart Inside the Chest." pp. 125-130 (1962); B. Vajapeyam, "A Microcomputer Based Control System for the Artificial Heart," Trans. ASAIO, Vol. XXV, pp. 379-382 (1979); M. Arobia et al, "A New Automatically Controlled Electric TAH," Trans. ASAIO, Vol. XXVI, pp. 60-65 (1980); V. I. Shumakeov et al, "Control System for an Artificial Heart," translated from Meditsinskaya Tekhnika, No. 4, pp. 22-29 (July-August), Plenum Publishing Co.; F. Klines et al, "Analysis and Model for Controlling System to Control Heart Rate and Stroke Volumes of an Aritficial Heart," Medical and Biological Engineering, pp. 662- 668, September 1975. On the other hand, there is evidence from natural heart transplants that such control is unnecessary; transplanted human hearts have no neural connections to the host body, and hence their cardiac output is only indirectly related to neural control; yet people with such transplants have been able to lead meaningful lives. It may be concluded that a TCP can be satisfactorily operated without such control. The evidence above teaches that a workable TCP can be made to approximate the natural heart's Starling's Law behavior with relatively simple control operations.
Thus, a TCP is now technically feasible, provided that a competent design is constructed. The critical blood pumping technology appears to be well established and adequate for long-term survival of the recipient. Benign power transmission across the skin can obviate the portent of infection of the thoracic cavity transmitted via percutaneous leads. One major area where satisfactory progress has recently been made is in the development of a blood pump and blood pump actuating mechanism as set forth in the prior related Pat. Nos. 4,369,530 and 4,376,312. Another major area where satisfactory progress has been lacking, however, is the provision of a reliable and effective electronic control system for a TCP. This latter objective is the one to which the present invention is principally addressed.